Rolled or extruded aluminium Al-Mn alloy products with improved corrosion resistance

ABSTRACT

The invention concerns a rolled or extructed product, in particular a tube, made of an alloy composition (expressed in wt. %) comprising: Si&lt;0.30; Fe 0.20-0.05; Cu&lt;0.05; Mn 0.5-1.2; Mg&lt;0.05; Zn&lt;0.50; Cr 0.10-0.30; Ti&lt;0.05; Zr&lt;0.05; the balance consisting of aluminium and unavoidable impurities. The invention also concerns a method for making extruded tubes of said composition comprising casting a billet, optionally homogenizing it, extruding a tube, drawing said tube in one or several passes and continuous annealing at a temperature ranging between 350 and 500° C. with a temperature increase of less than 10 seconds. The inventive products are designed for pipes and heat exchangers for motor vehicles, and exhibit good corrosion resistance.

FIELD OF THE INVENTION

The invention relates to rolled or extruded aluminium Al—Mn alloyproducts (series 3000 according to the Aluminium Associationnomenclature) with improved corrosion resistance, and particularly totubes intended largely for pipes or tubes and strips for use in heatexchangers for motor vehicles.

DESCRIPTION OF RELATED ART

The most frequently used alloys for tubes to be used as pipes for motorvehicles, and for tubes and strips for use in mechanically assembledautomobile heat exchangers are manganese 3102, 3003 and 3103 alloysaccording to standard EN 573-3. These alloys are suitable for extrusionand have satisfactory mechanical properties. However, it becamenecessary to improve the corrosion resistance of these applications,which lead to the development of “long life” alloys.

Patent application WO 97/46726 by Reynolds Metals applies to an alloyknown as X3030 with composition (% by weight):

Mn 0.1-0.5; Cu<0.03; Mg<0.01; Zn 0.06-1.0; Si 0.05-0.12; Fe<0.50; Ti0.03-0.30; Cr<0.50, the balance consisting of aluminium. The addition ofZn and Ti contributes to improving the corrosion resistance. Cr ispreferably held below 0.20% and the examples have contents of 0.005%,0.05% and 0.10%. Fe is preferably kept below 0.30% and the examples havecontents of 0.10%, 0.12% and 0.20%.

Patent application WO 99/18250 by the same company applies to an alloyknown as X3020 with better formability than X3030 by the addition of Mg(up to 1%) and Zr (up to 0.30%). Cr is preferably kept below 0.02%, oreven 0.01%. Fe is preferably kept below 0.30%.

Patent application WO 00/50656 by Norsk Hydro relates to an alloy withcomposition Si 0.05-0.15; Fe 0.06-0.35; Cu<0.10; Mn 0.01-1.0; Mg0.02-0.60; Cr<0.25; Zn 0.05-0.70; Ti<0.25; Zr<0.20.

Cr is preferably kept below 0.15% and is only allowed to enablerecycling of manufacturing waste from other alloys. Fe is preferablykept below 0.22%.

SUMMARY OF THE INVENTION

The purpose of the invention is to further improve the corrosionresistance compared with existing “long life” alloys.

The invention concerns a rolled or extruded product, in particular atube, made of an alloy composition (expressed in wt. %) comprising:

Si<0.30; Fe 0.20-0.50; Cu<0.05; Mn 0.5-1.2; Mg<0.05; Zn<0.50; Cr0.10-0.30; Ti<0.05 Zr<0.05; the balance consisting of aluminium andunavoidable impurities.

The preferred contents are Si<0.15; Fe 0.25-0.40; Mn 0.8-1.0; Cr0.15-0.30; Zn 0.10-0.25.

The invention also concerns a method for making tubes extruded of saidcomposition comprising casting a billet with the above composition,optionally homogenizing it, extruding a tube, drawing said tube in oneor several passes and continuous annealing at a temperature rangingbetween 350 and 500° C. with a temperature increase of less than 10seconds and preferably less than 2 seconds.

The invention also concerns a method for making strips with thiscomposition, comprising casting a plate, hot rolling and then coldrolling it, and continuous annealing at a temperature ranging between350 and 500° C. with a temperature increase of less than 10 minutes, andpreferably less than 2 minutes.

DETAILED DESCRIPTION OF THE INVENTION

The alloy according to the invention has a manganese content of the sameorder of magnitude as the 3003 and 3103 alloys and contains almost nocopper or magnesium. Unlike what is believed in prior art whichrecommends low iron contents for corrosion resistance, the behaviour ofthe alloy is good with an iron content of more than 0.20% and up to0.50%. This avoids the need to use a more expensive pure aluminium base.Also unlike prior art, a chromium content of more than 0.10% andpreferably 0.15% is shown to be favourable to corrosion resistance.Addition of a low content of zinc, less than 0.50% and preferably lessthan 0.20%, is also favourable.

The method for making extruded product, comprises casting the billets ofthe indicated alloy, homogenizing the billets, heating and extrudingthem to obtain a straight or ring tube, and one or several drawingpasses to bring the product to the required dimensions. The tube is thenannealed by taking it through a passage furnace, preferably an inductionfurnace, at high speed. The extruded product is heated very quickly, inless than 10 seconds and preferably in less than 2 seconds, and theproduct advances at a speed of between 20 and 200 m/mn. The furnacetemperature is kept between 350 and 500° C. The product can then bedrawn again to increase the mechanical strength.

This continuous annealing gives a microstructure with fine equiaxialgrains, a medium grain size as measured by the intercepts method of lessthan 40 μm and typically of the order of 30 μm, while with conventionalannealing in a static furnace, the grains are elongated with a muchlarger grain size of the order of 180 μm in the extrusion direction and70 μm in the direction perpendicular to extrusion. The presence ofchromium tends to further reduce the grain size by increasing thedensity of the recrystallisation sites, which has a beneficial effect oncorrosion resistance.

The method for making strips comprises casting a plate with acomposition according to the invention, optionally homogenizing thisplate, hot rolling this plate to obtain a strip, cold rolling this stripto the required thickness and final recrystallisation annealing to atemperature ranging between 350 and 600° C. This final annealing ispreferably done continuously in a passage furnace with a temperatureincrease in less than 10 minutes and preferably less than 2 minutes,which gives a fine grain size. The hot rolled strip may also be obtaineddirectly by continuous casting, for example by casting between twocooled cylinders.

The products according to the invention have better corrosion resistancethan the 3003 and 3103, and at least as good as “long life” alloysaccording to prior art, mechanical properties and formability equivalentto 3003 or 3103, and an economical production cost. They have anelectromechanical corrosion potential practically identical to that of3003 or 3103, such that there is no difference in behaviour of galvaniccouples, for example the tube-fin couple in heat exchangers. Theextrudability of extruded products is also at least as good as theextrudability of 3003 or 3103 alloys.

Extruded products according to the invention can be used particularly asfuel, oil or brake fluid pipes for automobiles and as tubes for heatexchangers in engine cooling systems and air conditioning systems forpassenger compartments in motor vehicles. The strips may be used in thesame heat exchangers for header pipes, evaporators and fins.

EXAMPLE

Billets made of 6 alloys identified A to F were cast and homogenized.The compositions of alloys A, B and C were the same as the compositionsof 3003, 3103 and X3020 according to prior art. Alloys D and E aredifferent from the invention in that D has a lower iron content and Econtains titanium instead of chromium. The alloy F is conform with theinvention. The compositions of the alloys (% by weight) are shown intable 1.

TABLE 1 Ref. Si Fe Cu Mn Mg Cr Zn Ti A 0.09 0.19 0.15 1.05 — — — — B0.12 0.55 — 1.10 — — — — C 0.07 0.08 — 0.95 — — 0.17 0.14 D 0.08 0.10 —1.00 — 0.20 0.01 0.02 E 0.07 0.26 — 0.98 — 0.01 0.01 0.14 F 0.08 0.27 —0.98 — 0.19 0.17 0.01

The billets were extruded in coiled tubes and were then drawn to obtaina diameter of 12 mm and a thickness of 1.25 mm. No significantdifference in terms of extrudability or drawing was found for any of thesix alloys. These coiled tubes were continuously annealed in aninduction furnace at a fixed temperature of 470° C. with an advancespeed of between 60 and 120 m/min. The rings were then drawn once againto bring them into the H12 state according to standard EN 515. Theultimate strength R_(m) (in MPa) and the yield stress R_(0.2) (in MPa)were then measured on the samples of the 6 tubes. The results are givenin table 2.

TABLE 2 Alloy A B C D E F R_(m) 116 120 106 112 112 116 R_(0.2) 102 11395 106 98 108

It can be seen that the mechanical strength of the alloy according tothe invention is almost exactly the same as the alloy according to 3003(A) or 3103 (B).

The corrosion resistance was measured using the SWAAT (salt water aceticacid test) test according to standard ASTM G85. Measurements were madefor three different cycle times, 100, 400 and 800 cycles on two 200 mmtubes per alloy and per duration. At the end of the test, the tubes weretaken out of the containment and were pickled using a 68% concentratednitric acid solution to dissolve corrosion products. The depth of the 5deepest pits formed was then measured on each tube, and the average foreach tube and the average P_(moy) of the values obtained for the twotubes were calculated. The corrosion resistance is better when P_(moy)is smaller. The results are given in table 3.

TABLE 3 Alloy A B C D E F 100 cycles 273 220 101 91 164 91 400 cycles462 375 213 134 151 124 800 cycles 1054 431 368 260 387 249

It can be seen that alloy F according to the invention is the alloy thatgives the best results, and therefore the iron content of 0.27% does notreduce the corrosion resistance, and the addition of 0.20% of chromiumfor alloys D and F has a significantly beneficial effect.

1. A rolled or extruded product, made of an alloy consisting essentiallyof, in weight %: Si<0.30; Fe 0.25-0.50; Cu<0.05; Mn 0.5 1.2; Mg<0.05;Zn<0.50; Cr 0.10-0.30; Ti<0.05; Zr<0.05; the balance aluminum andunavoidable impurities.
 2. Product according to claim 1, whereinSi<0.20%.
 3. Product according to claim 1, wherein Fe is between 0.25and 0.40%.
 4. Product according to claim 1, wherein Mn is between 0.8and 1.0%.
 5. Product according to claim 1, wherein Cr is between 0.15and 0.30%.
 6. Product according to claim 1, wherein Zn is between 0.10and 0.25%.
 7. Product according to claim 1, which is an extruded productwith grain size less than 40 um.
 8. A pipe for a motor vehiclecomprising an extruded product according to claim
 1. 9. A tube for anautomobile heat exchanger comprising an extruded product according toclaim
 1. 10. An element for an automobile heat exchanger comprising astrip according to claim
 1. 11. Product according to claim 1, which is atube.